In mobile applications, circuits are increasingly required to be more power efficient and are required to run off an ever decreasing power supply. Unfortunately, not all circuits function well with low power supplies, including pixels in an image sensor. In order to compensate for a lower power supply, an on-board voltage doubler is often used, which creates higher voltage supplies for circuits such as pixels. A common form of voltage doubler is the charge pump.
Charge pumps are noisy circuits. They work by doubling a reference supply, and so noise on this reference is effectively sampled, and doubled. In order to make a quieter supply, the reference supply is often regulated, so that the sampled noise is less. This has the drawback that there is a high current overhead required as the regulator should be able to supply large current spikes.